Typically, voltage regulator circuits provide a constant output voltage of a predetermined value by monitoring the output and using feedback to keep the output constant. FIG. 1 shows an output driver 1 of a typical prior art voltage regulator. A PNP pass transistor 12 has its emitter coupled to an input voltage rail 14 and its collector coupled to an output terminal 16. Pass transistor 12 is typically a high power transistor. Voltages at rail 14 typically range from 5 to 30 volts.
A PNP transistor 18, operating as a current source, is connected to the collector of an NPN transistor 20. The collector of NPN transistor 20 is also coupled to the base of pass transistor 12.
NPN transistor 20 is controlled by a control signal coupled to its base 21 to turn on pass transistor 12 by pulling down the base of pass transistor 12. Diode 22, which is typically a diode-connected transistor, and resistor 24 are connected between the base of transistor 12 and voltage rail 14 to provide a pull-up voltage for the base of pass transistor 12.
Pass transistor 12 provides a variable current to output terminal 16 for driving a load (not shown) coupled between output terminal 16 and ground. The voltage at output terminal 16 is regulated by a feedback loop (not shown) which adjusts the control signal to the base 21 of NPN transistor 20. When the load requires a large current, the feedback circuit increases the control voltage to transistor 20 which, in turn, increases the conductance of pass transistor 12 to provide the necessary collector current to drive the load at output terminal 16. Conversely, when the load requires only a small amount of current, a lower control signal is generated by the feedback circuit so that pass transistor 12 provides only a small amount of current to output terminal 16. When no load current is desired, the control signal turns off transistor 20. However, when a small load current or no load current is intended to flow, relatively small base-substrate and base-emitter leakage currents within pass transistor 12, as well as similar leakage currents in transistor 20, result in unwanted additional current flowing through the base of pass transistor 12. This undesirable base drive current further turns on transistor 12 to some extent, causing a larger load current to be provided at output terminal 16 than was intended.
The amplification (or beta) of pass transistor 12 is problematic for another reason. At low base currents the beta of pass transistor 12 is higher than at larger base currents. Since the impedance looking into a transistor's base is proportional to its beta, it follows that the pass transistor's base impedance will also increase at low base currents. This larger base impedance, together with parasitic base-substrate and base-emitter capacitances of pass transistor 12, forms a low pass filter at the base of transistor 12. This low pass filter adds an additional pole to the regulator circuit, thereby limiting the frequency response of the circuit and causing unwanted oscillations at the output terminal 16.
The above-described problems are largely overcome by diode 22 and resistor 24. First, by providing a relatively low impedance path between the base and emitter of pass transistor 12, diode 22 and resistor 24 dominate the base impedance of pass transistor 12 at low base currents. The resultant decrease in base impedance mitigates the effects of the low pass filter, thereby improving the frequency response of the circuit and reducing the likelihood of oscillations.
As mentioned earlier, when no load current is to be provided, transistor 20 and pass transistor 12 should be in their off states. When transistor 20 is in its off or low conductivity state, the current through diode 22 and resistor 24 pulls up (i.e., pulls to a higher voltage) the base of transistor 12 to a high enough voltage to force transistor 12 to turn off despite the above-described leakage currents. In this manner, diode 22 and resistor 24 offset the tendency of leakage currents to turn on transistor 12 when no or little load current is to be provided to the load. Thus, by offsetting the base drive current of transistor 12, diode 22 and resistor 24 prevent undesirably large DC bias currents from flowing when little or no load current is desired.
At high input voltages and high temperatures, however, diode 22 and resistor 24 cease to operate properly in keeping transistor 12 turned off when no load current is to be provided. As the input voltage and/or the operating temperature increases, pass transistor 12's base-substrate and base-emitter leakage currents rapidly increase to levels sufficient to turn on transistor 12. As the input voltage exceeds approximately 40 volts, pass transistor 12 begins to experience punch-through, which causes a large collector current to flow. This large collector current causes the collector (or output) voltage to increase beyond its regulated level. This higher collector voltage, in turn, further increases the undesirable base drive discussed above. Diode 22 is no longer able to pull-off the base of transistor 12 hard enough or quickly enough to offset the strong base drive current resulting from the leakage currents. As a result, at such high temperatures and input voltages pass transistor 12 will supply a large collector current to the load. Such a large collector current, as discussed above, is problematic when the load requires no current or a small current.
Thus, there is a need for an improved output driver which reduces its associated pass transistor's base impedance at high input voltages and temperatures. There is also a need for an output driver which has the capability of pulling-off (i.e., turning off) its associated pass transistor at high temperature and high input voltage without any increase in ground current when the pass transistor is off.